This is cute, they're a real idiot couple. Though I wonder how (it appears) humans are the top dogs when it seems mermaids are superior in every way; physical strength, claws, are able to be on both land and sea without ships, based on her necklace and bracelets it's clear they can work metal and would be able to make weapons - why haven't they tried to conquer humanity yet?
Well, most mermaids can't go on land. She is special in that regard. It's apparently extremely rare. This was mentioned in chapter 1. So they really have no way of conquering humanity.
As for metalworking, her necklace and bracelets were made by humans and put on her after her capture. The bracelets are the remains of the shackles she was wearing in chapter 1. The neckpiece was probably made to match. She was dressed up since she was supposed to be a present for a king. Not to mention, metalworking would be incredibly difficult underwater.
The other reason is probably numbers. Generally, the more powerful the individuals of a species are, the less of them there are.
Also, it's possible that they are all extremely dumb. Though it could just be her.